One issue that has emerged during the nuclear crisis in Japan is that there isn't always a reliable source for radiation levels from specific areas. RDTN.org has just launched, an experiment to help address that need. The site allows people to submit their own reads, and maps them out next to data from official sources and measurement dates. This way, anyone can quickly get an idea of what is happening on the ground, first-hand. The site is brand new but should be very useful going forward.
Also worth noting and specific to what is going on in Japan right now, JapanStatus.org is "a dashboard of accurate, sourced information on the situation in Japan following the March 2011 disaster."